neurophysiology

glia have only been studied

the last 15-20 years

glia fuction

shape the NS environment by
1. modifying neuron communication by altering ion balances (shaping the ionic environment)
2. wrapping around axons providing myelination
3. monitoring synapses and re-uptake mechanisms
4. are the resident immune cells of the CNS

microglia

the resident immune cells of the CNS, that climb into the brain during embryo development

glia of the CNS

astrocytes, oligodendrocytes, microglia

macroglia

astrocytes and oligodendrocytes

PNS glia

Schwann cells

developmental glia

radial glia - that are actually stem cells that the NS forms from

oligodendrocytes structure

have many small branches that reach out into the extracellular environment

oligodendrocytes function

myelinate fibres in the CNS, explore and wrap around fibres giving the correct signals

how do oligodendrocytes make myelin?

extend cell membrane and wrap tightly around axons - 30-40 layers of a 2 layer membrane

____ of axons in the CNS are myelinated

10%

each oligodendrocyte

myelinates many axons, and vice versa

oligodendrocytes special function

can suppress regenerating axons in the CNS, through myelin blocks, making repair and regrowth after injury impossible
this is to preserve pre-existing structures, and the lifetime of experience stored in the brain

Schwann cells

myelinate and maintain some axons and isolate them from the external environment, with a combination of oligodendrocyte and astrocyte functions

Schwann cells provide a pathway

supportive pathway for regenerating peripheral nerves

how does myelin give us an advantage?

it is very compact and allows for very fast signal transmission, giving vertebrates an advantage

astrocytes are named for

their star-like appearance - of their internal cytoskeleton

cytoskeleton of astrocytes

set of branching, radiating proteins that hold the structure of the cell

astrocytes actual shape

complex fluffy shape surrounding neurons - has infinitesimally small branches and leaflets radiating outwards, and very fine processes to fill every nook and cranny of the CNS

functions of astrocytes

metabolic support (nutrients and waste products), maintaining the blood brain barrier (everything must pass through an astrocyte prior to entering the CNS), talk to neurons, influencing their activity, receiving and sending neurotransmitters, exciting and inhibiting neurons, and neurotransmitter re-uptake/recycling

each astrocyte…

maintains its own domain in the CNS, are extremely territorial and regulates cell environment of dozens of neurons

neurons signal

rapidly from point to point

astrocytes signal by

sending waves via gap junctions which affect large volumes of tissue, in the form of calcium waves through the astrocyte network, independent of neuron activity

blood brain barrier

maintains integrity of CNS and what can get through, consists of astrocyte endfeet wrapping around capillary walls, controlling which water soluble molecules can get in/out of the brain

water soluble molecules

waste, nutrients, oxygen, CO2, drugs

fat soluble molecules

dissolve in membranes and can pass to and from the brain easily - this includes alcohol and most psychoactive drugs

microglia

the defence system of the CNS, resident immune cells that are typically quite

microglia activate

in injury and inflammation, and make inflammatory responses to injury

microglia revert to

phagocytic, mobile forms that activate, engulf, and eat intruders

microglia may have a role in

neurodegenerative diseases, in multiple sclerosis as a part of the immune response attacking myelin, in Alzheimer’s when cell death occurs, cleaning up debris

neurons

rapid and reliable signalling across long distances in the body, detecting significant events, identifying interesting information, and shaping the NS to respond to the environment to survive by organising and controlling our movement and behaviour

input zone

dendrites, soma

dendrites

receive input

soma

cell body and support centre of the cell, contains nucleus and sends signals via thte axon to axon terminals

axon hillock

conducts the automatic process of weighing up responses, the AP beginning here - it is the trigger zone and has a lot of voltage gated channels

axons are

very long and require lots of maintenance

cell cytoskeleton provides

transport and structure

cytoskeleton consists of

actin and microtubules and neurofilaments (MT and NF for transport)

conducting zone

axon

output zone

axon boutons

action potential

a standard, self-regenerating signal which replicates across an entire cell, unchanged from start to finish, acting as a standardised membrane switch

maintaining a state…

of readiness uses lots of energy, but individuals do not, allowing neurons to sustain firing for long periods of time without loss of quality

all APs are

2 sets of states the parts of the cell jump quickly between that influences the rest of the cell, all the APs being fundamentally identical, information is conveyed by timing of APs and speed

cell membrane

made up phospholipid bilayer and specialised proteins that carry things through the membrane

at rest

dominated by potassium ions

at APs

dominated by sodium ions

cell membrane is an excellent

insulator, preventing flow of ions, and capacitator, holding the charge of the membrane

channels allow

electric current flow via ions

gating channels

types of proteins, some always open, other gated, and are selective for ion types or charge types

parts of AP

excitatory synaptic potential pushes cell over threshold, voltage gated sodium channels depolarise the membrane, voltage gated potassium channels depolarize the membrane

local current flow

occurs between active and adjacent inactive areas, causing other parts of the axon to become depolarized

synapses

major form of connectivity in the CNS

smart communication of synapses

both sides have a say in synaptic communication, conversations that may or may not change behaviours

plasticity

the effect of the synapse can be modified

better than a digital computer

allows for real time processing of thousands of input, each neuron is subject to 2000-5000 influences at one time

reasons to use synapses

smart communication, allows for plasticity, better than a digital computer

components of a synapse

presynaptic terminal, synaptic cleft, postsynaptic membrane

presynaptic terminal

activity is signalled by releasing neurotransmitters to the post-synaptic cell

synaptic cleft

the space neurotransmitters must cross

post-synaptic membrane

has neurotransmitter receptors

NTs do not

enter the receiving cell - they attach to receptors on the cell membrane/dendrite and act as a lock and key, opening receptors

NTS need to be

cleared away, by being broken down by enzymes, taken up by glia, or being pumped back into the presynaptic terminal

sum of inputs

excitatory and inhibitory synapses combine their influences to determine is the cell will fire, and can combine by being close together in location or timing

NTS

actual molecules that are used to send signals

brain excitatory NT

glutamate

brian inhibitory NT

GABA and some glycine, suppress depolarization

SC and PNS excitatory

glutamate for neurons, acetylcholine for muscles

SC and PNS mixed

acetychloline in ANS and PSNS, noradrenaline in SNS

SC and PNS inhibitory

mostly glycine, some GABA

noradrenaline

vigilance, attention by exciting activity of cerebral cortex and shifting patterns of activity

serotonin

very complex regulation of mood and sensory integration, with subtle regional function

dopamine

reward (do we want to do this again), pleasure, movement control, affects plasticity

acetylcholine

attentio, wakefulness, memory, cognition, biases system but does not directly direct them

sensation

the detection of things in the outside world by neurons able to capture certain kinds of energy

our sensory experience is

constructed in the brain from the decoding oftime impulses from millions of receptor cells

impulses are decoded by

which afferent fibre carries them, timing of activity regarding speed and attention

abstract interpretation

our brain is stuck in a skull and does not experience anything itself, relying on receiving signals from sensory organs scattered around the body

during development

our brain recognises and enhances order and patterning in its input, combining and interpreting those ordered signals into a rich, synthetic experience

sensory receptor

a cell able to detect a physical stimulus and create a signal through transduction, the converting the energy

a receptor responds…

to one kind of energy, and signals its presence, or some aspect of the stimulus, using APs in axons or cells

types of sensory coding

rate, population, latency

rate coding

firing rate describes intensity, frequency, or some other property

population coding

different stimuli excite different groups of receptors

latency coding

different stimuli take different time to trigger the receptor

electrophysiology

how we listen to neural code, the most common approach to studying sensory systems by presenting artificial stimuli while recording responses

receptive fields

most receptors will only respond to a stimulus in a particular location, a receptive field - the region of space where stimuli can affect the firing of a receptor

neurons further along the communication…

communication change integrate many receptors’ activity into complex higher receptive fields or perceptual experiences with additional properties, while retaining spatial organisation and distribution

during development (RF)

the brain and spinal cord learn and make connections between parts of receptive fields, allowing the brain to make inferences and decode input easier

simple responses connect

to higher order cells which combine inputs in many complex ways, creating higher order mental processes

stimulus processing

the typical ways the NS looks for useful information - centre surround, lateral inhibition, contrast detection

centre surround

the background influences responses to a stimulus

lateral inhibition

stimuli compete with their neighbors for attention

contrast detection

NS respond most where there is an abrupt transition in properties

sensory pathways

3 major routes travelled by sensory information in the CNS - special senses, somatosensation, visceral sensation

special senses

olfaction, visio, gustation, audition, vestibular travel via cranial nerves to brainstem and midbrain

somatosensation

skin/deep tissue enters via spinal nerves and some cranial nerves, travelling via spinal cord circuits and ascending tracts

visceral sensation travels via

autonomic afferent nerves - vagus nerve and spinal nerves

olfaction is transmitted by

1st cranial nerve, small branches passing through piriform plate at the top of the sinus

olfactory receptors

touch air in the nasal cavity and have a large variety of receptor proteins for specific odorant molecules, and the presence of the right molecule in the air activates the protein, allowing for smell

olfactory bulb

tract in the NS, travelling from the 1st cranial nerve region to other locations